It's not. You are just taking the definition of "Nuclear Family" in a vacuum. It's like thinking "Abolish the Police" means that everyone that says that means they want anarchy, no state, and no form of law enforcement.
It's a general phrase that speaks to the current structures and their real material results.
Your "idea" of the police or the nuclear family may be their definition on paper. An armed law enforcement. Or a single man, single women having kids.
These are what they are defined as in a vacuum. They are not what they are in the context of state structures.
The police are a defense of the capitalist class. Their job is first and foremost to protect private property and ensure class structures are not threatened.
The Nuclear Family is a means of ensuring a women's material conditions and her children are dependent on a man for his labor. That same structure ensures men are given the advantage of control in that family structure; while keeping them having no power in their work or their labor.
These are what "leftist" are talking about when we are talking about abolishing these systems.
It's not about being benevolent. It's about being educated in class struggle and patriarchy.